pH and DO control at pharma company

 

A biopharmaceutical company asked DotX to provide:
  1. On-site assistance in applying the PID Tuner to tune 4 control loops in two 2000 L Bioreactors
  2. On-site Training in PID Tuning using the DotX PID Tuner software

In this post, we explain how we worked, and the amazing results.

 

The control challenges

The Bioreactor under consideration is a modern high tech piece of equipment with PID loops for pH, Dissolved Oxygen (DO) and temperature. The most challenging to control is DO. This is due to the fact that DO is controlled by a mix of gasses, including oxygen, CO2, and air. The PID output of the DO loop is the input for a schedule of these gasses.
 

Analysis of the PID loops

We analysed the control schemes, and data of the last batch. From our analysis, we found:
  • that all PID loop tunings could be improved with at least a factor 2 (in terms of closed-loop response time, and damping)
  • there was an issue with the schedule for gasses that are used to control DO: at one point, this schedule was causing unstable loop behavior
  • it was useful to add low pass filtering to the DO signal, so as to avoid spikes when applying D-action in the PID controller

 

Result with DO loop

Below, you see measured data of the DO loop before optimisation (collected in a batch before), and after (collected in the batch after changing the PID settings. Clearly, around t= 75 hours, the deviations from target have reduced by more than a factor 10. This is partly due to the changes in the gas schedule, and the improved PID tuning.

 
DO_loop
 

Result with pH loop

Below, you see measured data of the pH loop before optimisation (collected in a batch before), and after (collected in the batch after changing the PID settings. Clearly, the closed loop response time has improved welll over a factor 4, damping has improved, and the deviations from target have reduced by more than a factor 3.

 
pH_loop